Leptotes cassius, commonly known as the Cassius Blue or Tropical Striped Blue, is a butterfly of the Lycaenidae family. It is found in Florida and the Keys, Texas south through the Caribbean, Mexico, and Central America to South America. Strays can be found in New Mexico, Kansas, Missouri and South Carolina. The wingspan is 20–35 mm. The butterfly species has an important role in Marisha Pessl's 2006 novel Special Topics in Calamity Physics. The protagonist, Blue van Meer, is named in honor of the Cassius Blue.